Dealkylation of Lignin to Phenol via Oxidation-Hydrogenation Strategy

Article abstract of DOI:10.1021/acscatal.8b00886

Lignin is a renewable and abundant aromatic polymer found in plants. We herein propose a "cutting tail" methodology to produce phenol from lignin, which is achieved by combining Ru/CeO₂ catalyst and CuCl₂ oxidant via an oxidation-hydrogenation route. Phenol was obtained from separated poplar lignin with 13 wt % yield. Even raw biomass, such as poplar, birch, pine, peanut, bamboo willow, and straw, could be converted into phenol in 1-33 mg per gram of biomass.

Full text of DOI:10.1021/acscatal.8b00886

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Article
Dealkylation of lignin to phenol via oxidation-hydrogenation strategy
Min Wang, Meijiang Liu, Hongji Li, Zhitong Zhao, Xiaochen Zhang, and Feng Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00886 • Publication Date (Web): 15 Jun 2018
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ACS Catalysis
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Dealkylation of lignin to phenol via oxidation-hydrogenation strategy
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Min Wang,† Meijiang Liu,†,‡ Hongji Li,†,‡ Zhitong Zhao,† Xiaochen Zhang† and Feng Wang*,†
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† State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of
Chemical Physics, Chinese Academy of Sciences, Dalian 116023 (China)
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‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China
ABSTRACT
Lignin is a renewable and abundant aromatic polymer found in plants. We herein propose a “cutting
tail” methodology to produce phenol from lignin, which is achieved by combining Ru/CeO2 catalyst and
CuCl2 oxidant via an oxidationꢀhydrogenation route. Phenol was obtained from separated poplar lignin
with 13 wt % yield. Even raw biomass, such as poplar, birch, pine, peanut, bamboo willow and straw,
could be converted into phenol in 1ꢀ33 mg per gram of biomass.
KEYWORDS: Biomass • Lignin • CꢀC bond cleavage • Catalysis • Phenol
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Introduction
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Lignin, an aromatic polymer, consists of phenylpropanoid building blocks which is majorly linked by
C–O/C–C bonds. It potentially serves as a renewable source for producing aromatic chemicals.1ꢀ2
Recent interests are directed to the cleavage of βꢀOꢀ4 bonds to aromatics via Cβ–O/Cα–Cβ bond cleavage,
such as solvolysis,3ꢀ5 pyrolysis,6ꢀ7 reduction,8ꢀ23 oxidation,24ꢀ37 and redoxꢀneutral methods.38ꢀ40 However,
these methods generally produce phenols mixture with C1ꢀC3 alkylꢀtails (including alkyl, alkene, and
oxygenꢀcontaining functionality) on the para sites. It is difficult to separate out pure products. Solving
this problem may rely on innovative strategy that can afford relative narrow product distribution.
Alkylꢀfree phenols, such as phenol, guaiacol and 2,6ꢀdimethoxyphenol, are useful chemicals.
Particularly, phenol is the key component of resin, bisphenol A and prolactam, and its world
consumption was up to 7 million tons per year.41 The production of phenol from lignin, instead of
cumene oxidation,41 is very interesting in the viewpoint of sustainability and economics, but also
challenging due to the presence of alkyl tails. Cutting the alkyl tails via Cary–Cα bond cleavage not only
leads to the fragmentation of lignin polymer, but also results in the formation of alkylꢀfree phenol
products, which is rarely explored. Sels and coworkers reported the conversion of 4ꢀpropylphenol to
phenol and propene.42 Cary–Cα bond in βꢀOꢀ4 ketone models was cleaved via Baeyer–Villiger (BV)
oxidation, while forming a new Cary–O bond.43 Cary–Cα bond in phenolic βꢀOꢀ4 models was oxidatively
broken up to generate benzoquinones.44
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We herein propose a route for the preparation of alkylꢀfree phenols by the combination of Cα–Cβ
bond oxidative cleavage to acids and subsequent decarboxylation (Scheme 1). We show that the Cary–Cα
bond in phenolic lignin models was facially cleaved in CuCl2 aqueous solution. The Hꢀbased βꢀOꢀ4
synthetic polymer was converted into phenol in 75% yield. By combination of C–O bond
hydrogenolysis over Ru/CeO2 catalyst and Cary–Cα bond oxidative cleavage, poplar lignin was converted
into phenol with up to 13 wt % yield. Some raw lignocellulosic biomass feedstocks were converted into
phenol with 1ꢀ33 mg per gram of biomass.
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ACS Catalysis
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Scheme 1. The production of phenol from lignin.
Experimental section
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General information. RuCl3 was purchased from Shenyang Nonferrous Metal Research Institute.
Active carbon and SiO2 were purchased from Macklin. ZSMꢀ5 was purchased from Nankai University.
CeO2 were prepared according to our previous work.45
Preparation of supported Ru catalyst. Ru/C, Ru/ZSMꢀ5, Ru/ZSMꢀ5 and Ru/SiO2 and Ru/CeO2 were
prepared by an incipientꢀwetness impregnation method. RuCl3 was used as the ruthenium source. The
Ru content of Ru/C, Ru/ZSMꢀ5, Ru/SiO2 was 5 wt %. The Ru content of Ru/CeO2 was 2 wt %.
Typically for the preparation of Ru/CeO2, 0.002 g of RuCl3 and 0.5 g of CeO2 was dispersed in 20 mL
of deionized water, and magnetically stirred for 24 h. And then the water was evaporated by heating at
100 °C. The resulting solid was reduced at 400 °C (5 °C minꢀ1) by H2 (5 mL minꢀ1 H2 and 10 mL minꢀ1
N2) for 2 h.
Synthesis of lignin models. βꢀOꢀ4 alcohols and ketones were prepared according to our previous
work.25 The βꢀOꢀ4 polymers were synthesized according to the previous report.46 The detailed
procedures were provided in the supporting information.
Reaction procedure. The catalytic reactions were performed in a 10ꢀmL autoclave reactor with an
internal Teflon insert. Typically, 0.2 mmol of lignin model, 2 mL of 0.25 M metal salts were added into
the stainless steel autoclave with an internal Teflon insert, and then heated to 200 °C under magnetic
stirring in Ar atmosphere. For the reaction of lignin extracts and raw biomass, 100 mg of substrate and
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0.01 mmol of Ru catalyst was added and 3 MPa of H2 was charged into the reactor. The procedure for
the reaction of raw biomass was the same as lignin extracts except that 0.2 mmol of AlCl3 was added.
After the reaction, the water solution was extracted with 40 mL of ethyl acetate five times (8 mL × 5),
and the asꢀobtained solution was concentrated to about 2 mL in a rotary evaporator under reduced
pressure at 313 K. A certain amount of pꢀxylene was added into the above solution as internal standard.
And then, the products were analyzed and quantified by gas chromatography−mass spectrometry
(GC−MS) using an Agilent 7890A/5975C instrument equipped with an HPꢀ5 MS column (30 m in
length, 0.25 mm in diameter). For the analysis of the acid product, the acid product was first converted
into the corresponding ester and then analyzed. The procedure was as follows: the solvent of extracted
solution was removed in a rotary evaporator under reduced pressure at 313 K. The asꢀobtained residues
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were dissoved in 5 mL of methanol and 50 ꢁl H2SO4 . The solution was heated at 100 C for 6 h under
Ar atmosphere. The yield was determined using internal standard method and the selectivity was
determined via peak area percentage method. Column chromatography was employed to separation of
phenol. Ethyl acetate and petroleum ether with 1:2 ratio was used as an eluent.
Extraction of poplar lignin. To poplar sawdust (40 g) was added 1,4ꢀdioxane (280 mL) followed by 2
mol L–1 HCl (36 mL) and the mixture was heated to a gentle reflux (390 rpm minꢀ1) under a N2
atmosphere at 120 °C for 2.5 hour. The mixture was then allowed to cool down and the lignin
containing liquor was collected by filtration. The collected liquor was partially concentrated by
evaporation to give a gummy residue which was reꢀdissolved in 1,4ꢀdioxane (10 mL) and then
precipitated by addition to rapidly stirring water (250 mL). The crude lignin was collected by filtration
and dried under vacuum. The dried crude lignin was dissolved in 1,4ꢀdioxane (10 mL) and precipitated
by dropwise addition to rapidly stirring Et2O (250 mL). The precipitated lignin was collected by
filtration and dried under vacuum to give a purified poplar lignin (6 g). This lignin was used in
subsequent experiments without further processing.
Results and discussion
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ACS Catalysis
Firstly, we used pꢀhydroxybenzoic acid to study the decarboxylation step in aqueous solution at 200 °C
for 6 h under Ar atmosphere (Figure 1). The decarboxylation reaction did not occur without catalyst.
Copper catalysts are known to be good for decarboxylation reaction.47 We then screened copper
catalysts, including CuCl, Cu(NO3)2, Cu2O, CuO, Cu, CuSO4 Cu(OAc)2 and CuCl2 in this reaction.
Although all these copper catalysts could offer phenol at a 20 mol % adding amount, CuCl2 showed the
highest phenol yield with 48%. The anion may act as a ligand to affect the electronic properties of
copper. We have characterized the copper salts by linear sweep voltammetry (LSV) (Figure S1). The
onset reduction potential of CuCl2 is 0.25 V (vs SCE) which is much higher than Cu(OAc)2, CuSO4 and
Cu(NO3)2, which may account for the higher activity. With excess amount of CuCl2, the phenol yield
was increased to 79%. CO2 was detected in the gas phase (Figure S2a). Water is a better solvent than
dioxane and methanol. pꢀHydroxybenzoic acid was converted to pꢀhydroxybenzoate in methanol solvent
which prohibit the decarboxylation reaction. The carboxylic acid groups is easier to dissociate to form
carboxylate anion in water which may be the reason for the better performance in water solvent.
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Figure 1. The decarboxylation of pꢀhydroxybenzoic acid. Reaction conditions: 0.2 mmol of pꢀ
hydroxybenzoic acid, 20 mol % of copper catalyst, 2 mL of water, Ar, 200 °C, 6 h. The results were
determined by GCꢀMS. a 0.5 mmol of CuCl2. b Methanol as solvent. c Dioxane as solvent.
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Table 1. Substrate scope.a
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a Reaction conditions: 0.2 mmol of substrate, 2 mL of 0.25 M CuCl2 aqueous solution, Ar, 200 °C, 6 h.
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The results were determined by GCꢀMS. Isolated yields. c 50 mg of 2 wt % Ru/CeO2 was added and
under 3 MPa of H2 atmosphere. d 0.1 M CuCl2 aqueous solution.
The presence of phenolic group in the reactants is beneficial for the decarboxylation reaction.
When 4ꢀmethoxybenzoic acid was used as substrate, the decarboxylation reaction could not proceed and
only 2% yield of anisole was obtained (Table 1, entry 2). The presence of phenolic groups facilitates the
formation of oxocarbonenium ion intermediates via reaction with the acidic site (Figure S3). The
electronꢀwithdrawing ability of oxocarbonenium ion can stabilize the negative intermediate and thus
enhances the decarboxylation reaction. When a hydrogenation catalyst, such as Ru/CeO2, was used in
H2 atmosphere, 4ꢀmethoxybenzoic acid was converted into phenol in 80% yield (Table 1, entry 3). pꢀ
Hydroxybenzoate, which is presence in lignin as PB unit, can also converted into phenol in 60% yield
(Table 1, entry 4).48 Vanillic acid and syringic acid were also converted into guaiacol and syringol with
55 and 75% isolated yields, respectively (Table 1, entry 4 and 5).
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The phenol building blocks are dominantly connected via βꢀOꢀ4 linkages. Then, we investigated
the combination of Cα–Cβ bond cleavage and decarboxylation for the βꢀOꢀ4 cleavage. It is undesirable
to carry out the reaction in oxygen atmosphere because phenols are unstable in oxygen atmospheres
especially under high temperature.25ꢀ28 In our previous work, we found copper as a redox metal can be
used as oxidant for the cleavage of C–C bond in the absence of oxygen.28 Here, we used CuCl2 as both
oxidant for the Cα–Cβ oxidative cleavage and catalyst for the decarboxylation. The synthetic phenolic βꢀ
Oꢀ4 polymer was broken down to phenol in CuCl2 solution with 75% yield [eq. (1)]. CuCl2 was reduced
to copper as indicated by the Xꢀray diffraction characterization (Figure S4), and HCl was generated,
resulting in the decrease of pH value from 5.4 to 2.5. Nonꢀphenolic βꢀOꢀ4 dimer with H unit underwent
Cα–Cβ bond cleavage to acid and phenol [eq. (2)]. However, the Cary–Cα bond could not be broken via
decarboxylation due to the absence of phenolic group. The βꢀC was converted into CO2 as evidenced by
the detection of CO2 in the gas phase (Figure S2b). Previously, we have demonstrated the oxidative
cleavage of βꢀOꢀ4 is via βꢀOꢀ4 ketone intermediates.25ꢀ28 Nonꢀphenolic βꢀOꢀ4 ketone with CγꢀOH were
also underwent Cα–Cβ bond cleavage to acid and phenol [eq. (3)], indicating that the cleavage of βꢀOꢀ4
dimer is probably via βꢀOꢀ4 ketone. In the absence of Cα–OH hydroxyl group, no reaction happened [eq.
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(4)]. Our previous works showed that the Cα–Cβ bond in βꢀOꢀ4 ketone linkage with G and S units is
more difficult to be oxidatively cleaved due to the steric and electronic effect.25 The present method is
selective for the cleavage of Hꢀbased βꢀOꢀ4 monomer and show low efficiency in the cleavage of Sꢀ and
Gꢀbased linkages [eq. (5)].
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Scheme 2. Proposed reaction mechanism for the dealkylation of phenolic βꢀOꢀ4 linkages.
Based on the above results, a possible route for the dealkylation of βꢀOꢀ4 linkage is proposed
(Scheme 2). As demonstrated by our previous work,25ꢀ28 the Cα–Cβ bond in βꢀOꢀ4 ketone is easier to be
broken than that in βꢀOꢀ4 alcohol. When 1ꢀphenylethanol was used as probe molecules, acetophenone
was detected as intermediates (Figure S5). The oxidative cleavage of C(OH)ꢀC bond is probably
proceeded via C(=O)ꢀC intermediate, that is, oxidation of Cα–OH to ketone first takes place, and then
oxidative cleavage of Cα–Cβ bond results in the formation of pꢀhydroxybenzoic acid. The Cα–Cβ bond
cleavage is probably via hydroxyl ketone structureꢀlike intermediate which is formed via the CβꢀH bond
oxidation.25,49 CuCl2 as oxidant is reduced to copper, releasing HCl. Finally, CuCl2 catalyzes the
decarboxylation of pꢀhydroxybenzoic acid to phenol. The αꢀC and βꢀC was converted into CO2 as
evidenced by the detection of CO2 in the gas phase (Figure S2).
Then, we tried to depolymerize organosolv lignin. The lignin was extracted from the lignocelluosic
source via acid treatment in the dioxane solvent. Phenol in 2.6 wt % yield was obtained as the major
product from poplar lignin, together with 0.2 wt % yield of guaiacol and syringol. As proved above,
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without phenolic groups, no dealkylation occurred. To further increase the phenol yield, hydrogenation
catalysts were used to create more phenolic groups. With supported Ru catalyst and under hydrogen
atmospheres, the yields of phenol were all increased (Figure 2a). Ru/CeO2 with 1 nm Ru cluster (Figure
S6), prepared according to our previous work,50 showed the best performance and offered 13 wt % yield
of phenol along with 2 wt % and 0.5 wt % yields of guaiacol and syringol, respectively. The selectivity
of phenol was up to 60% (Figure 2b and S7a). As shown above, the βꢀOꢀ4 linkage with G and S units is
more difficult to be cleaved due to the steric and electronic effect, which explains the formation of a
small amount of Gꢀ and Sꢀbased monomer. Instead, with Ru/CeO2 as a catalyst and in the absence of
CuCl2, no phenol was obtained and a mixture of products, including substituted phenols and saturated
compounds, was produced (Figure S7b).
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Figure 2. (a) The dealkylation of extracted poplar lignin over supported Ru catalyst in CuCl2 aqueous
solution. (b) The product distribution over Ru/CeO2, which was determined via peak area percentage
method. Others are the aliphatic. Reaction conditions: 100 mg of poplar lignin, 0.01 mmol of Ru, 2 mL
of 0.25 M CuCl2 aqueous solution, 3 MPa of H2, 200 °C, 6 h.
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The role of Ru/CeO2 is the hydrogenolysis cleavage of R–OAry bond to phenolic group and thus
facilitates the decarboxylation reaction. With the addition of Ru/CeO2 and under hydrogen atmosphere,
pꢀmethoxybenzoic acid was converted into phenol in 80% yield (Table 1, entry 3). Me–OAry bond was
cleaved over Ru/CeO2 to form phenolic group, resulting in the decarboxylation of pꢀhydroxybenzoic
acid to phenol over CuCl2. By combination of Ru/CeO2 and CuCl2, pꢀmethoxyl substituted lignin model
was converted to phenol. The selectivity of Cary–Cα bond cleavage was 77% (Figure 3).
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Figure 3. The dealkylation of nonꢀphenolic lignin models over Ru/CeO2 in CuCl2 aqueous solution.
Reaction conditions: 0.2 mmol of model, 50 mg of 2 wt % Ru/CeO2, 2 mL of 0.25 M CuCl2 aqueous
solution, 3 MPa of H2, 200 °C, 6 h.
Next, we tried the direct conversion of raw biomass, which majorly consists of lignin, hemicellulose
and cellulose. The lignin twines with hemicellulose and cellulose, forming complex and robust structure,
hampering it to be accessed by solid catalyst, and therefore is a more challenging task. The oxidationꢀ
hydrogenation strategy via combination of Ru/CeO2 and CuCl2 is applicable to the raw biomass
resource (Table 2). AlCl3 was added to hydrolyze cellulose to release lignin. The lignin was converted
to phenol, and cellulose was majorly converted to levulinic acid (LA) and γꢀvalerolactone (GVL)
(Figure S7c). Various raw biomass sources, including poplar, pine, birch, peanut, bamboo willow and
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straw, were tested, and 1ꢀ33 mg of phenol, 70ꢀ224 mg of LA and 2ꢀ48 mg of GVL per gram of biomass
were obtained, respectively. The poplar showed the highest yield of phenol with 33 mg gꢀ1.
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Table 2. The conversion of raw biomass. a
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Entry
Lignin source
Phenol / mg gꢀ1
LA / mg gꢀ1
GVL / mg gꢀ1
1
2
Poplar
Pine
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Birch
Peanut
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Bamboo willow
Straw
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a
Reaction conditions: 100 mg of substrate, 50 mg of 2 wt % CeO2, 0.2 mmol of AlCl3, 2 mL of 0.5 M
CuCl2 aqueous solution, 3 MPa of H2, 200 °C, 6 h. The results were expressed as mg product per gram
of biomass (mg gꢀ1).
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It should be noted that besides the three primary building blocks of H, G and S units, there are
some other units, such as pꢀhydroxybenzoate (PB), which is similar to the H unit and can be the
resource of phenol. As shown in Figure 4, the pꢀhydroxybenzoate (PB) unit, such as pꢀhydroxybenzoic
acid, pꢀmethoxylbenzoic acid and pꢀhydroxybenzoate, were effectively converted into phenol with
around 80% yields (Table 1, entry 1, 3 and 4). It is reported that poplar lignin is rich in PB unit.48 We
also characterized the poplar lignin and birch lignin by 2D HSQC NMR (Figure 4). The content of PB
unit is up to 17.1% for poplar lignin, while birch lignin contains only 2.1% PB unit. This accounts for
the higher yield of phenol from poplar lignin. Because poplar lignin contains 17.1% PB unit and H unit
was not detected, the formation of phenol majorly comes from PB unit via ester bond cleavage.
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Figure 4. 2D HSQC NMR characterization of poplar and birch lignin.
Conclusion
In summary, we reported an oxidationꢀhydrogenation strategy for conversion of lignin to phenol by
combination of Ru/CeO2 and CuCl2. Ru/CeO2 hydrogenolyzes C–O bond to release phenolic group and
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CuCl2 accounts for the oxidative cleavage of Cary–Cα bond. This strategy can be used in converting
lignin extracts and native lignin. The ligninꢀbased process is a promising and renewable route for the
production of phenol
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ASSOCIATED CONTENT
AUTHOR INFORMATION
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Corresponding Author
*Eꢀmail: wangfeng@dicp.ac.cn
Supporting Information. Detailed experimental procedures, the synthesis of lignin models, some
experimental data, NMR spectra and GCꢀMS spectra. “This material is available free of charge via the
Internet at http://pubs.acs.org.”
ACKNOWLEDGMENT
This work was supported by the National Natural Science Foundation of China (21603219,
21690082), DICP (DICP ZZBS201613), the Strategic Priority Research Program of Chinese Academy
of Sciences (XDB17020300), Department of Science and Technology of Dalian under the contract of
2017RQ114 and Department of Science and Technology of Liaoning province under the contract of
2015020086ꢀ101.
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